Wide-angle lens attachment with front element of high index glass



Jan@ ze., 1952- Filed Sept. l, 1950 y K. TOLLE WIDE-ANGLE LENS ATTACHMENT WITH FRONT ELEMENT OF HIGH INDEX GLASS 2 SHEETS-SHEET l Jan. 8, "1952.

K. TOLLE v WIDE-ANGLE LENS ATTACHMENT WITH FRONT ELEMENT -oF HIGH INDEX GLASS Filed sept. l, 195o 2 SHEETS- SHEET 2` Patented Jan. 8, 1952 UNITED STATES vAMTYENT oFFlcE 'Y WIDE-*ANGLE LENS ATTACHMENT. WITH v FRONT ELEMENT or man INmaxxdLAss Karl Tolle, Rochester, N. Y., assignor-to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jerse1 f Application September 1, 1950, Serial No. 182,699

l 'I'his invention relates to wide-angle attachments for photographic objectives. j v

'I'he object of the invention is lto provide a substantially afocal wide-angle attachment which is more highly corrected for distortion and field curvature than heretofore available and which is highly corrected for spherical aberration and coma, and especially adapted for use'with highaperture objectives around f/1.4.

It is Well known to place a reversed Galilean telescope in front of a photographic objective to cover a wider object eld than the objective alone is capable of covering. By reversed isv meant a telescopic system having its negative member in front, i. e. facing the distant object, and its positive member behind and adjacent the objective. Such an attachment has a mag- 5 Claims. (Cl. 88---7-57) ,k

niflcation less than unity and usually between 0.25 and 0.9. VThat is', the equivalent focal length of the combination is between 0.25 and 0.9 times f the equivalent focal length of the objective alone.

Such telescopic attachments are usuallyafocal,

that is the distance from the rear principal pointl of the front member to the front principal point of the rear member is equal to the numerical difference between the focal lengths of the two members, so that very distant objects are focused at the same distance behind'the objective whether or not the attachment is used. It may be preferred in some conditions of use, however,

`to adjust the members slightly laway from the afocal spacing for focusing and in particular to adjust them so that the image distance corresponding to some nearer .object remains unchanged when the attachment is added to or removed from the objective, but the attachment is still approximately afocal.

In known afocal wide-angle attachments the Lnegative member is usually meniscus in 'shape and convex to the front, but there is considerably more variety in the shapes of the positive member.

According to the present invention, a substantially afocal wide-angle attachment for photographic objectives is made up in which the two members are concave toward each other, the

" negative member consists of a front positive element and a reary negative element cementedA thereto, the refractive index of the front positive element is between 1.75 and 2.0, and the difference between the index of the front element and that of the second element is between 0.23

and 0.50. The curvatures ofthe three surfacesy of the front member depend somewhat upon the magnification and are preferably within the limits set forth by the following algebraic inequalities:

where M is the magnification of the attachment,

Fn is the focal length` of the positivemember.

fumar-p1)` and (51m-4).

'andRL Ri and R3 Iare the'radii of curvature of the three surfaces of the front member in order vfrom front to rear. For example, when the magnification is 0.5, F11/R1 is between +l.2 and +3.0, F11/R2 is between 2.4 and 0.8, and F11/R3 is between +6.5 and +8.?, and when the magnication is 0.8 the corresponding ranges are +3.0 to +4.13, zero to +1.6 and +7.4 to +96 respectively. Positive and negative values of curvature, indicate surfaces respectively convex and concave to the front, and a zero value indicates a -plane surface.

. The curvature of the cemented surface is chosen to correct the distortion and ycorresponds closely to curvatures knownvin the prior art.

`A large index diierence at this surface, how- 'in that of the negative member. I have found it advantageous to make the positive` member menisous in outward form, that is concave toward thefrontand convex toward the rear, and preferably the curvature l/Rk of the rearl surface is such that F11/Rt is numerically between The positive member may satisfactorily be made of a simple positive element .of low dispersion glass when the magnification is between 0.9 and about 0.7 or even 0.6 if the requirements of lateral color correction are not too critical. An achromatic doublet is preferable if the vcolor is to be highly corrected. For practical reasons, the doublet is y preferably cemented but may bemade up with a small airspace. An airspaced doublet provides an additional tool for the designer to. correct coma more easily, but is generally considered more expensive to, make than the cemented l doublet.

.The refractive indices of the glass or glasses making up the positive member may be varied considerably without having a very great effect on the spherical aberration, coma, distortion or curvature of field. Accordingly they are chosen so as to control the Petzval sum to some degree and thereby to control the astigmatism. The

stronger attachments, that is those whose magnication is smaller and which accordingly Vchange the focal length more, tend to have a more negative Petzval sum, and likewise the. iPetzvalsum tends to be more negative if an at- *t'achment is redesigned in amore compact form, "'that'isdf' the focal lengths of thetwo members are individually decreased vvhile being maintained in the same ratio toeach other. This tendency toward a negative Petzval sum can ibe.-

counteracted to a large degree by making up. the. positive member of one or more -highindex neg-V ative elements and one or more low-index positive elements. Known glasses suitable for use;

- muchlessand it is often advisable to designan attachment to givethe best results with a particular objective and rto modify. the design for use Vwith a differentobject-ive.-

The curvature ofthe front surface of the rear member varies greatly` with the indices of the .glassesfofwhich this member is made up, but it is preferably concave. When therefractive .indei:` is..substantially .uniform throughout this memben. .this-curvature is preferably numerically between. (5M 014). and A(5M4e3), vanglvvhen the average. index of; the negative elementv or ele` ments'exceeds that of the-positive elementY or elements. by. AN `it is preferably.v between (5M -.O.4.1.-.8AN and (5M+3.+8AN).. The-.curvaturesof. vthe interior surfaces of this component,

if. any,V are, deGermi. 11e.d by the requirements of colorcorreetion.

inthe.accompanyinedrawinssr. v Y Figs.. 1 and 2 show an afec/al attachment acboardingV to theinvention. and datafor. a specific example thereof; Y Figs. 3 and. llf-sholvv a simpler form ofthe invention and dataV for a speciiic. example thereof;

Figs.v 5 and. 6: show asIightlydiHerent. form of the/invention and data for. .a specic. example thereof; l

Figs., '7. and show a further variation of the invention and.. data for a, specific. example thereof.

In each.y of. Figs. 1f, 3.-, 5, vand 7, an axialdiagranry of the wide-angle. attachment is. shown Y irl-full lines; andthe objective Ill that. i-t s-fattached infront of is represented inoutline by .abroken line. Y 1

The. tables of data. given in Figs. 2, 4, Gand 8 are repeated below for convenience, and along lwith themv additional examples are given. In eaeh-tablfe, the lenselements are numbered in the first;columnA in order. from front. to rear.; Jthe second andthirdcommns give thecorrespond- .ing refractive,V indicesN, for theD line ofthe lspectrum and the-,dispersive indicesY. The last twocolumns-sive, the, radii. of. curvature R of the lenssurfases, thethicknesses 1 or. the lens elements and the axial distancesA s betweenfcomponente., each manneredV bysubseripts in order f rom f..ront,..to rear. The el ar1.d -,r values. of4 the radii denotesurfaces respectivelyA convex.. and concaveto. the. front.- AlsofFNse-,andFPos dengtefthefoeal lengths of the. negative andthe positive rrlellbers1 respectively. All the examples I are given-on assaler according to. Which-,the focal length.- -Eposof thefpositive1 member -is 10.0 mm.

-Thef magnification is then 0.01times` the f oeal length Qi the negative member.

Example-$1. shown in Figs., 1 andi-, consists of twof eementedfdoublets.. each consisting; of, `two meniscus elements. That. is; in this case-,.1 the f twov cemented; surfaces.; are concave towardfeaeh 4 otherf' The .specications for this example are as follows:

Example 1, Figs. 1 and 2 `Exfr'nple y3, shownin Figs. 5,.and.:6, has asome- -what stronger .wideangle effeet. that visa .lower magnfleaton. vand the cemented. .surface `in .the nent-component. is concaveto. the front in accordance withV theinvention when'applied 120 strongery attachments. The cemented. surface lof the rear member is also curved inthe opposite direetionfrom that shown. in Example 1,.but I do not Yconsider this. to be. an essentia1. featurefof theinve'ntion... Thisis a. buried.surface, vthat is. therefractive index isI substantially.v the same on .both sides.. so. that. ,its curvature. can be varied during the. design Oia System, for correctingcolor in known. manner. andv without. disturbing the other corrections. The. data for this example is as iollows:

Ezvample'3. Figs. 5 and-t 6 -Exampleseand 5. aresimilar to Example 3 Abut usedifferentztynes. of glassfin thetwo front elev.ments and the. magnicationis.l somewhat different in. Example 5.. Theirspecincationsare as t8 Example 5, Fig'. 5 '.vature `ofthis surface is much weaker than in the F 7M F :m0 l.other examples by about`8AN/Fn, where AN'V is NBG mm m mm the index difference and'Fir is the focal length of Lens N V Rami i Thickucsscs 'fthe positive meinber.-v The specifications for this 5 example are as. follows: v "i 1 .=4.57 .r. I' 2 1.24 .8.8 mm 2- ...mm @M -m7855188- .7 @mi 8 R3= +1270 81=22.71 .i 1 FNEG=62 4 mm;V Fp05=100.0mm 1.. 3 1575 41.4 Ri= 22.55 :1 1.50 n 4 1.525 57.4 R5= +80.42 74= 4.47 Y v r e. Rei 17? 10 Lens H N V Radii f Thicknesses Example 6 differs but little from Example 3 in 1 1.880 l.41.1 if +o48i5mm. gli respect to the radii of curvature and is made up 2 11500, SL16 y Rig-#Iggy ,122757 of the same kinds of glass. It is included to .3 1.804. 41.8.. gi S illustrate the invention embodied in an attachgw .4 1 .523' .5&6 R; 4.34114 1; 3105 i. ment designed to be mounted at agreater disy R1= -18.5 7 Y tance from the standard camera objective. All the other examples are intended to be mounted .Y with their respective rear surfaces within 3 mm. .Examples 1 .2 3' 4 and Wer? Ongmany de signed for use in combination with 25 mm. obof the front surface of the standard obJective,

\ Jectives and have been made up with the focal 'but Example 6; was deslgned to be mounted 20 len ths of the ositive members ranging from mm. in front of the objective when the shape of 112gmm in Exanlple 4 to 169 mm in Example 2 the standard lens mount prevents the mounting Examples 5 and 6 were originally designed for .of the attachment closer than this. The ceuse in front of 13 mm. obJectives and with the mented surface in the fi ont doublet has a weaker focal lengths of the positive members 67 mm. curvature, that is it 1s varied in the plus direcand 79 mm respectively tion, to restore the distortion, lateral color, and In accordance with te invention curvature of field corrections after this change. Otherwise it is very similar to Example 3. The (GM- Fn/Ri) Speclcatlons are as follows: '30 is algebraically between zero and +1.8,

Example 6, Fig. 5 (8M Fn/R2) f F Mmmm P0" 1000mm is between +48 and +64, (8M-Fume) is .be- .Lens N V Rad Thcknesses tween 5.0 and 7.2, and the front and back surfaces of the positive member are concave toward the front. Also in accordance with a prei 1.880 41.1 .R 50.80 1 5.40 2 1.500 01.0 R;=-80.4 n lm 2= 1.92 mm ferred feature of the invention, (5M-|-F1i/R74Vis 3 1 575 4M jg algebraically between -1 and 4 in each ex- 4 1.572 57.4 R5= +88. 02 a= 8.20 ample, where R8 is the radius of curvature of RF "2037 '-40 the rear surface of the positive member. With this radius in the preferred range, the curvature Example 7 Shown 1n F1`gS- 7 andfls 9 m0d1 I/R4 of the front surface of the positive niem- .Caf/{On 0f Example Exmlfle 3 1S Partlclllal'y ber is largely determined by the refractive indesigned to work with obJectives having slightly dices used in the positive member and by the H lWald @I J-IVmg eld, Whelfea Example 7 1S Spe-"45 focal length Fn assigned thereto. In each of clflcauy mtended for 'ObJCtWeS hafVPg a Per' the above examples this curvature is such that fectly flat eld. one way in which 'the field 5M 8AN+F/R4 is between 3 and +0.4, might be @Orreted would nVOlVe Chaflgrlg R1 where AN is equal to zero when thepositive and R3 but this would introduce astigmatlsmmember consists of a single element and isequal Instead, H1 EXamI-Jle 7,. the glaSS 111 1611s 3 1S Ie- 50 rto the difference (N3-N4) between the .refracplaced by a glass 0f hlgher 1n deX tqchanee the -tive index N3 of the negative element of the pos- PetZval Sum in the P1115 dI'eClOn- FOI 6601101115. jitive member and the refractive index N4 0f the the Sa-me identical flOnt doublet is llSed in bOth `posit-,ive Velement', 0f the positive member When designs, and under this restriction a small airthe positive member is a substantially achrospace is introduced in the rear doublet to `cor- 55 matic doublet. The value of -AN can be negarect both coma and spherical aberration simultive, but it is more usually positive. taneously. Due to the high index difference there The numerical values of these several quanis considerable negative power in the internal tities are summarized in the following table:

Example No l 2 3 4 5 6 7 Magnification 784 787 624 .624 714 626 624 l Fu/ +3.07 +8.40 +2.00 +208 +2.82 +1.07 +2.00 Fup/R2 +0.80 +0.72 0.59 0.08 0.27 0.78 0.09 F11/R +9.15 +7.85 +7.80 +7.83 +7.87 +7.78 +7.80 10i/R1-- 5.99 4.17 8.90 8.50 4.44 3.01 1.58 F11/R 7.20 5.82 4.9i 4.09 5.00 4.91 5.39 N .003 .000 .009 .008 .003 .008 .281 F11/R 6M 1.08 1.32 1.74 1.71 1.40 1.79 1.74 Fn/Rq-SM 5.97 5.58 5.08 5.07 5.98 5.74 5.08 I/R-s +0.80 +5.49 +4.98 +5.40 +5.78 +5.90 +5.93 Fn/R4+0M 8 AN... 2.09 0.24 0.80 0.40 0.89 0.80 0. 7i F/R,.+5M 8.84 1.89 1.79 1.57 2.09 1.78 2.27

pair of surfaces, so that the front surface R4 needs less negative power to maintain the focal length of this component. Also due to the higher index of lens 3, this front surface R4 needs less In every case in which the positive member is made up of two elements, the curvature of the pair of internal surfaces (usually cemented together), as determined by the requirements of curvature for any selected power, hence the curcolor correction, is such that Fn times this curvsuchthat (M-i-Fn/Rk) isbetween Nl and of the positive member.

the curvatures of the two surfaces.

Wide-angle attachments according to the invention have been made up and proven to be very satisfactory in use. It is contemplated that the invention will bemost useful in attachments having magniiications between 0.9 and about 0.45.

The focal lengths given above for Examples 2 to 5 inclusive are computed for the actinic rays (the g or G wavelength) Vand those given for Examples 1, 6 and 7 are. computed for the visual rays (the D wavelength). are only a fraction of one percent, however I claim:

l. A wide-angle attachment for use on the `front of a photographic objective comprising a meniscus negative member axially aligned with and approximately afccally spaced in front of a ,positive member which is meniscus in outward -form and concave toward the front, in which the negative member consists of a front positive -element and a negative element cemented to the are such that (6M F11/R1) isrbetween zero and `-{-1.`8, (SM-Fn/Rz) is-between 4.8 and 6A, and y(.aM-.Fn/Rgl is between 5.0 and 7.2, wherein Fu is the focal length of the positive member, M is the magnification ofthe attachment and is -between 0.45 and 0.9 and where a radius of curvaturehas a positive value when the surface is convex tothe front and a negative value whenthe surface is concave to the front.

2. A wide-angle attachment according to claim 1 in which the radius of curvatureRk of the rear` surface of the positive member is negative and is A wide-angle attachment according to claim 2 in which the positive member consists of a negative element and a positive element in which that (v5M--8N3-k8N4+Fu/R4) is between -3 and i +04, where N3 and N4 are respectively the refractive indices of thenegative and positive elements .4. A wide-angle attachment for use von the front of a photographic objective comprising a .meniscus negative member axially alignedr withv The disparities, if any,

negative member inorder from front to rear, the radii of curvature- R4 andfRk of the front and rear surfaces of the positive member, and the air-space S betweenthe members are within the limits set forth in the following table of algebraic inequalities:

where Fpos is the focal length of the positive member, where and values of R denote surfaces respectively convex and concave tothe front, and where an infinitely large value of either sign denotes a plane surface.

5. An approximately afocal wide-angle attachment for use lon the front of aphotographic objective which consists of a front negative cemented doublet and a lrear positive cemented doublet coaxially spaced therefrom and which has substantially the specifications set forth in rwith the space s between the doublets and the focal length Fr of the negative, doublet, and where Fn is'the focal length .of the positive doublet and .the and evalues of the radii denote surfaces vrespectively convex and concave toward the front.

KARL TOLLE.

REFERENCES CITED The, following references are of reordin the file of this patent:

UNITED STATES PATENTS 

